RFR(XS): 8030976: untaken paths should be more vigorously pruned at highest optimization level

[Rickard Bäckman]

Updated: http://cr.openjdk.java.net/~rbackman/8030976.4/

Thanks
/R

On 06/03, Igor Veresov wrote:
> I didn’t add an entry in _trap_hist for Reason_tenured because it’s tracked by a different counter. Perhaps Reason_unstable_if can be moved before Reason_tenured so that _trap_hist_limit can be 21.
> 
> igor
> 
> On Jun 3, 2014, at 3:42 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> wrote:
> 
> > 2 questions:
> >  - why do you set _trap_hist_limit = 22 and not 21?
> > 
> >  - why do you map Reason_unstable_if to Reason_intrinsic in reason_recorded_per_bytecode_if_any?
> > +     else if (reason == Reason_unstable_if)
> > +       return Reason_intrinsic;
> > 
> > 
> > Best regards,
> > Vladimir Ivanov
> > 
> > On 6/3/14 5:07 PM, Rickard Bäckman wrote:
> >> Updated: http://cr.openjdk.java.net/~rbackman/8030976.2/
> >> 
> >> Thanks
> >> /R
> >> 
> >> On 06/03, Roland Westrelin wrote:
> >>> Hi Rickard,
> >>> 
> >>>> Updated webrev: http://cr.openjdk.java.net/~rbackman/8030976.1/
> >>> 
> >>> You need to pass Deoptimization::Reason_unstable_if to uncommon_trap() in Parse::adjust_map_after_if(). Also the code below in Parse::adjust_map_after_if() is no longer needed AFAICT.
> >>> 
> >>> Roland.
> >>> 
> >>> 1185     // If this might possibly turn into an implicit null check,
> >>> 1186     // and the null has never yet been seen, we need to generate
> >>> 1187     // an uncommon trap, so as to recompile instead of suffering
> >>> 1188     // with very slow branches.  (We'll get the slow branches if
> >>> 1189     // the program ever changes phase and starts seeing nulls here.)
> >>> 1190     //
> >>> 1191     // We do not inspect for a null constant, since a node may
> >>> 1192     // optimize to 'null' later on.
> >>> 1193     //
> >>> 1194     // Null checks, and other tests which expect inequality,
> >>> 1195     // show btest == BoolTest::eq along the non-taken branch.
> >>> 1196     // On the other hand, type tests, must-be-null tests,
> >>> 1197     // and other tests which expect pointer equality,
> >>> 1198     // show btest == BoolTest::ne along the non-taken branch.
> >>> 1199     // We prune both types of branches if they look unused.
> >>> 1200     repush_if_args();
> >>> 1201     // We need to mark this branch as taken so that if we recompile we will
> >>> 1202     // see that it is possible. In the tiered system the interpreter doesn't
> >>> 1203     // do profiling and by the time we get to the lower tier from the interpreter
> >>> 1204     // the path may be cold again. Make sure it doesn't look untaken
> >>> 1205     if (is_fallthrough) {
> >>> 1206       profile_not_taken_branch(!ProfileInterpreter);
> >>> 1207     } else {
> >>> 1208       profile_taken_branch(iter().get_dest(), !ProfileInterpreter);
> >>> 1209     }
> >>> 
> >>>> 
> >>>> Thanks
> >>>> /R
> >>>> 
> >>>> On 05/22, Rickard Bäckman wrote:
> >>>>> Roland pointed out a problem with the Reason used. New webrev coming
> >>>>> shortly.
> >>>>> 
> >>>>> Thanks Roland.
> >>>>> 
> >>>>> On 05/22, Rickard Bäckman wrote:
> >>>>>> Hi all,
> >>>>>> 
> >>>>>> can I please have reviews for this change.
> >>>>>> The patch makes C2 place uncommon traps on previously untaken branches
> >>>>>> much more aggressively (we are simply trusting the profiling more).
> >>>>>> This improves performance for a couple of different patterns.
> >>>>>> 
> >>>>>> Example:
> >>>>>> 
> >>>>>> class Test {
> >>>>>>  public int[] array = new int[] = { 1, 1, 2, 3, 5, 8, 13 };
> >>>>>> 
> >>>>>>  public void some_method() {
> >>>>>>    for (int i = 0; i < array.length; i++) {
> >>>>>>      if (array[i] < 255) {
> >>>>>>        some_call();
> >>>>>>      } else {
> >>>>>>        some_other_call();
> >>>>>>      }
> >>>>>>    }
> >>>>>>  }
> >>>>>> }
> >>>>>> 
> >>>>>> Where we previously if the else branch had never been taken rarely would
> >>>>>> inline the some_other_call and when array escapes we can't make
> >>>>>> assumptions on non-changing lengths, call killing registers, etc.
> >>>>>> 
> >>>>>> On some of the Nashorn benchmark this patch increases score by 35%,
> >>>>>> others don't see any change at all. No difference on SpecJBB 2005.
> >>>>>> More performance numbers / microbenchmark in the comments of the bug.
> >>>>>> 
> >>>>>> Webrev: http://cr.openjdk.java.net/~rbackman/8030976/
> >>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8030976
> >>>>>> 
> >>>>>> Thanks
> >>>>>> /R
> >>>>> 
> >>>>> 
> >>>>> /R
> >>> 
> 
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